1. Field of the Invention
The present invention relates to a signal sensor insensitive to variations of the static pressure reigning in the medium where it is disposed.
Such a signal sensor may be used in any medium in which it is desired to make acoustic signal measurements and particularly in water. It is suitable, in oceanography for example, for carrying out undersea detection operations or else for forming multi-sensor reception devices suitable for marine seismic prospection operations.
2. Description of the Prior Art
From the French Pat. No. 2 122 675, a hydrophone is known comprising two circular plates. One at least of the plates comprises a central flexible portion against which is fixed a sensitive element formed of a piezoelectric ceramic disk associated with an electrode on each of its two flat opposite faces. The peripheral portion of the two plates is reinforced and rigid. The two plates are applied one against the other and define an inner air-filled cavity. The variations of the seismic signals to be measured deform each flexible central portion. Each sensitive element operates under flexion, which gives it a high sensitivity. The static pressure which increases with the depth of immersion, for example, causes each flexible plate to bend inwardly of the case. The device is designed so that possible deformation of the flexible plates under the effect of the static pressure remains reversible. For this, the spacing between the two plates is chosen for example so that they abut one against the other before their deformation becomes irreversible.
Such a hydrophone is suitable for a given range of depths.
By making the two opposite plates thicker, the range of depths in which the hydrophone operates may be widened but to the detriment of its sensitivity. It should also be noted that the electric capacity of each sensitive element operating under flexion varies with the amount of flexion of the deformation. The response of the hydrophone changes then with the depth.
From the French Pat. No. 1 556 971, a hydrophone is also known comprising a sensitive element fixed to a thin flexible plate, which is secured to a rigid body. The latter has two inner air-filled cavities communicating through a capillary canal. A first cavity is separated from the external medium by a flexible membrane. The external surface of the flexible plate is exposed to the pressure to be measured. The pressure prevailing in the second cavity is applied to its internal face. The variations of the external pressure are transmitted into the first cavity by deformation of the flexible membrane. but the capillary canal acting as a low pass filter prevents the transmission of the dynamic variations of the pressure to be measured into the second cavity, from the inner side of the thin plate. The dynamic pressure variations may then be measured.
With such an arrangement, the range of depths in which the hydrophone may operate is appreciably increased. The limits are fixed essentially by the ability of the membrane to compensate, by its deformation, the variations of the external static pressure. It may also be noted that the dynamic variation filtering effect provided by the capillary, depends on its length. If it is desired to widen the passband of such a hydrophone as much as possible towards the low frequencies, a very long capillary must be used. In practice, such a solution is difficult to reconcile with the construction of very small hydrophones such as is used in large numbers for the construction of seismic streamers for example.